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I'm having lots of ideas for the use of Z-Unos. One project involves a 18V Ni-MH battery that is charged from time to time while the Z-Uno is connected.

Is this possible to connect directly? The Z-Uno is rated up to 18V so it should be ok, right?

Using a 12 V battery should be possible as well.

There are two issues.

Make sure you do not thermally overload the 5V regulator (NCP1117ST50T3G) by adding extra load. Check the power dissipation with a 13 V drop...

Also, I do not think charging it while it is connected is a good idea: 18 / 1.2 * 1.6 = 24 V. Abs max of the regulator is 20 V.

Edit: Maybe 1.6 V is too pessimistic, it depends on resistance, charge current, charger in use... But even with a typical 1.4 you get.. 21 V. As with all maximum ratings, it depends on temperature, how well "in spec" your chip is, ... So you might get away with this, but maybe not always

Ok, right now I have an Arduino Yun connected, which I plan to replace by ZWAVE. I think I have some power regulator connected to the battery. It works since years. But I guess I will continue using this part of the electronics and nit take the risk.

The device will get very hot in summer so heat could be an issue.

I agree. Also, from the top of my head, that 5 V regulator uses 5 - 10 mA. Is that OK for your application? On the other hand, the 3.3 V domain can go to nearly zero current...

As I remember the Yun draws over 300-400 mA, so the total of 40 mA of the Z-Uno is very welcome already. But it is possible that I have regulator for 3.3 connected right now (can't check this easily).

Do I need to do anything else or just connect 3.3V and the Z-Uno is more efficient?

You could remove R12 to squeeze out some uA but it will be pretty good if you power the Z-Uno's 3.3 V or even the 5 V is OK. The 5->3.3 regulator has low quiscent current.

Wait a second, I am not sure about the current on the 5 V regulator, I am not sure if it does not back-feed the NCP1117... Let me check that.

I have checked it, it is OK to supply power to the 5 V pin, the 5 -> 3 V regulator only adds 12 uA on my board at room temperature.
No need to remove R12, it is not there to "squeeze out some uA". By removing R12 you disconnect the 3 V regulator from the 3 V pin and the battery. So it no longer can operate on USB power but it also does not inadvertently charge your battery. On one of my boards I have removed R12 and soldered the output to a wire crimped to a JST PH 2.0 female plug. On the Z-Uno, I have soldered a 90° male PH connector. So now I can choose between battery power or the Z-Uno internal regulator and I am certain I never connect both at the same time.

Have fun with your (battery powered) Z-Uno!

Ok, so I can also power it by 3.3 V, right?

Yes, everything on your Z-Uno gets power from the 3 V pin.